Radio Frequency IDentification (RFID) tags can be used in many ways for locating and identifying objects to which they are attached. RFID tags are particularly useful in product-related and service-related industries for tracking large numbers of objects are being processed, inventoried, or handled. In such cases, an RFID tag is usually attached to individual items, or to their respective packages.
In principle, RFID techniques entail using a device called an RFID reader to interrogate one or more RFID tags. Interrogation is performed by the reader transmitting an interrogating Radio Frequency (RF) wave during a transmit phase. During a receive phase, a tag that has received the interrogating RF wave responds by transmitting back another RF wave, a process known as backscatter. The response may further encode a number stored internally in the tag. The response, and the number if available, is decoded by the reader, which thereby identifies, counts, or otherwise interacts with the associated item. The number can denote a serial number, a price, a date, a destination, other attribute(s), any combination of attributes, and so on.
An RFID tag includes an antenna system, a radio section, a logical section, and a memory. Advances in semiconductor technology have miniaturized the electronics so much that an RFID tag can generate the backscatter while powered by only the RF signal it receives, enabling some RFID tags to operate without a battery.
It is desirable that the antenna system have components such that it can sense many possible types of interrogating RF waves, and from many possible directions, regardless of the orientation of the tag. For example, some RFID tags are provided with antennas that are suitable for sensing RF waves of different polarization. It has been known to have a system of two antennas, receiving two RF signals from them, processing them, and then choosing the strongest one. Such is taught, for example in Patent Application US 2002/0167405A1, published on Nov. 14, 2002 to Shanks et al. An inefficiency, however, is where the two RF signals are processed independently, using one set of components for each of the two RF signals. This requires more circuitry and more power than single antenna systems.